US20050242991A1 - Method and apparatus for improved position, velocity, orientation or angular rate sensor - Google Patents
Method and apparatus for improved position, velocity, orientation or angular rate sensor Download PDFInfo
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- US20050242991A1 US20050242991A1 US10/835,699 US83569904A US2005242991A1 US 20050242991 A1 US20050242991 A1 US 20050242991A1 US 83569904 A US83569904 A US 83569904A US 2005242991 A1 US2005242991 A1 US 2005242991A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/13—Receivers
- G01S19/14—Receivers specially adapted for specific applications
- G01S19/15—Aircraft landing systems
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/13—Receivers
- G01S19/35—Constructional details or hardware or software details of the signal processing chain
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/13—Receivers
- G01S19/35—Constructional details or hardware or software details of the signal processing chain
- G01S19/36—Constructional details or hardware or software details of the signal processing chain relating to the receiver frond end
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/38—Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system
- G01S19/39—Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system the satellite radio beacon positioning system transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/38—Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system
- G01S19/39—Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system the satellite radio beacon positioning system transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/396—Determining accuracy or reliability of position or pseudorange measurements
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/38—Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system
- G01S19/39—Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system the satellite radio beacon positioning system transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/42—Determining position
- G01S19/45—Determining position by combining measurements of signals from the satellite radio beacon positioning system with a supplementary measurement
- G01S19/47—Determining position by combining measurements of signals from the satellite radio beacon positioning system with a supplementary measurement the supplementary measurement being an inertial measurement, e.g. tightly coupled inertial
Definitions
- the present invention is directed generally to navigation systems and, particularly, to an improved system and method for position, velocity, orientation or angular rate sensing.
- GNSS sensors are used in vehicles such as aircraft to determine vehicle position, velocity, orientation (attitude), and angular rate.
- GNSS position sensors can also be used to determine a velocity estimate by solving a set of range-rate equations or by smoothing (curve fitting) a set of position measurements.
- a GNSS position sensor typically includes an antenna and an RF coaxial cable coupling the antenna to a GNSS signal processing unit.
- the GNSS antenna generally includes an antenna element and associated filtering and amplification electronics. Position is sensed at the phase center of the antenna (typically close to the geometric center of the antenna element). Typically, GNSS sensing provides bandwidth up to about 10 Hz.
- GNSS orientation (attitude) sensors are also known (Orientation of a rigid body in space is defined by three (3) independent parameters. While various specifications of these parameters are possible, heading, pitch and roll are commonly used.).
- a GNSS attitude sensing system 100 is shown in FIG. 1A .
- a GNSS attitude sensing system includes a plurality of GNSS antennas 102 a - 102 d coupled via coaxial cables 104 a - 104 d to a GNSS receiver unit 106 .
- the relative positions of the antennas are used to derive a vehicle orientation.
- a GNSS attitude sensing system can generate angular rate measurements by solving a set of range-rate equations or by smoothing (curve-fitting) a set of attitude measurements.
- the antennas are attached to the receiver via coaxial cables.
- low-noise amplification (LNA) and filtering electronics are placed in the GNSS antennas 102 a - 102 d .
- the coaxial cable is used to transmit power from the receiver electronics to the antenna LNA electronics and to transmit the bandlimited GNSS signal to the receiver unit 106 , where further amplification, filtering and signal processing is performed.
- Inertial sensors such as accelerometers and angular-rate sensors, may be used either alone or in conjunction with GNSS sensors to determine changes in position, velocity, orientation, and angular-rate.
- Change in position for example, can be determined by twice integrating a set of accelerometer measurements; change in velocity can be determined by once integrating a set of accelerometer measurements.
- an angular-rate sensor can directly measure angular-rate. The change in orientation can then be derived from integrating the angular rate measurement.
- Inertial sensors are typically deployed in an inertial measurement unit (IMU) that houses, for example, an accelerometer, angular-rate, temperature and related sensors, as well as associated power supply, sampling filtering, and computational electronics.
- IMU inertial measurement unit
- the IMU is typically located close to the center of gravity of the vehicle; the mounting orientation within the vehicle is an important installation constraint.
- the system 100 includes an exemplary IMU 108 positioned generally at the vehicle's center of gravity and remote from the GNSS receiver electronics 106 .
- the measurements from both the GNSS sensors and the inertial sensors are available for processing.
- the GNSS measurements can be used to calibrate the inertial instruments over time by updating estimates of inertial sensor parameters at the relatively slow GNSS update rate.
- latency between the IMU processor and the GNSS processor will generally preclude calibrating in the reverse direction.
- the IMU may be combined with the GNSS receiver electronics in a single enclosure.
- a processing unit 105 includes both GNSS receiver electronics 106 a and IMU 108 a .
- This topology offers several advantages over the topology of FIG. 1A . These include elimination of the communications harness between the IMU and GNSS receiver unit; reduction in communication latency and complexity; and synchronous sampling of GNSS and inertial measurements, allowing: high bandwidth inertial measurement flow into the GNSS tracking channels; low bandwidth processed GNSS measurements update inertial measurement parameters; and high bandwidth GNSS phase data are available for update of inertial instrument parameters.
- the GNSS receiver 106 senses position (phase information) at the phase centers of its antennas.
- the IMU samples and integrates its internal sensors at the physical location of the IMU. Because the antennas and the IMU enclosure are physically separated, a projection algorithm must be applied before the measurements can be compared for purpose of complementary filtering.
- the projection algorithm requires the vectors between the IMU and the antenna phase centers be accurately known. This requirement can be problematic because it mandates an installation calibration procedure that may be complex; the vectors may change over time; and the vectors may change during operation, e.g., due to the structural flexibility of the vehicle, or elements of the vehicle.
- a global navigation satellite system (GNSS) receiver system includes a processing unit; and one or more antenna units for receiving GNSS signals, each of the antenna units having a phase center; one or more inertial sensor units each positioned substantially adjacent said phase centers; and at least one communication channel between each antenna unit and said processing unit.
- the processing unit uses data from the one or more antenna units and data from the one or more inertial sensor units to determine at least one of a position, velocity, orientation, or angular rate of the object.
- the antenna units include a single antenna element, the inertial sensing unit being positioned substantially adjacent the phase center of the antenna element.
- the antenna units include a plurality of antenna elements mounted on a substantially rigid substrate, and the inertial sensing unit is positioned substantially adjacent an antenna unit phase center, defined as the geometric mean of the phase centers of the antenna elements.
- rigidity of the substrate denotes relative motion between the antenna elements of less than about one hundredth of the highest frequency GNSS carrier wavelength during operation.
- An antenna unit for use in a global navigation satellite system (GNSS) receiver includes an integrated inertial sensor unit positioned substantially adjacent a phase center of the antenna unit.
- the phase center of the antenna unit is the phase center of a single antenna element.
- the phase center of the antenna unit is the geometric mean (equally weighted centroid) of the phase centers of a plurality of antenna elements.
- FIG. 1A and FIG. 1B illustrate GNSS and IMU systems according to the prior art
- FIG. 2A is a block diagram of an exemplary positioning and navigation system in accordance with an embodiment of the invention.
- FIG. 2B illustrates an exemplary object employing a positioning-navigation system according to an embodiment of the present invention.
- FIG. 3A and FIG. 3B illustrate exemplary antenna units according to embodiments of the present invention.
- FIG. 4 is a block diagram of an exemplary processing unit according to an embodiment of the present invention.
- FIG. 5 is a block diagram of an exemplary positioning and navigation system in accordance with an embodiment of the invention.
- FIG. 6 is a block diagram of an exemplary processing unit according to an embodiment of the present invention.
- FIG. 7 is a diagram illustrating operation of an embodiment of the present invention.
- FIG. 2A a block diagram of an exemplary navigation-positioning system 201 in accordance with an embodiment of the present invention is shown.
- the system provides global navigation satellite system (GNSS) based and inertial-based determination of vehicle position, velocity, orientation (attitude) and angular rate.
- GNSS global navigation satellite system
- Inertial sensors are co-located with GNSS antennas to more accurately derive the desired position and rate information.
- the GNSS receives positioning signals from the Global Positioning System (GPS), the system may be used with other radio based positioning or navigation systems, such as the GLONASS system, Galileo, or other systems such as pseudolites, low earth orbiting satellites (LEO), geosynchronous satellites, etc.
- GPS Global Positioning System
- the navigation-positioning system 201 includes a plurality of antenna units embodied as integrated GNSS Antenna-Inertial Sensing Units (GA-ISU) 206 a - 206 d .
- G-ISU Antenna-Inertial Sensing Units
- each GA-ISU 206 a - 206 d combines one or more GNSS antenna elements for receiving GNSS signals with an inertial sensor for determining, e.g., vehicle position, velocity, orientation (attitude) and angular rate. It is noted that while four such units are shown, in practice the number may vary.
- Each GA-ISU 206 a - 206 d is coupled to a GNSS Receiver-Inertial Measurement Unit (GPS-IMU) processor 202 .
- GPS-IMU GNSS Receiver-Inertial Measurement Unit
- the GPS-IMU processor 202 receives the GNSS antenna sensor data and the inertial sensor data, which it processes to derive the desired position, velocity, orientation and angular rate information. The resulting information is then provided to a navigation processor 203 for use, for example, in steering control or course guidance.
- FIG. 2B illustrates typical vehicle topology for a positioning-navigation system according to embodiments of the present invention.
- a vehicle 200 which may be embodied as an aircraft, although the invention is not so limited and may be implemented in any body whose position, orientation, flexibility, etc., is desired to be determined.
- the body may be a substantially rigid body; a substantially flexible body; or a plurality of substantially rigid, connected, bodies articulated (or independently moving) with respect to one another.
- a body may be considered to be flexible if due to structural flexibility, relative motion between affixed antennas of greater than one tenth of the highest frequency GNSS carrier wavelength is possible during operation. Otherwise, a body may be considered to be rigid.
- the GPS-IMU 202 and GA-ISU's 206 a - 206 d are fixed in suitable locations of the vehicle 200 .
- the GPS-IMU 202 is coupled via one or more coaxial cables 204 to the GA-ISUs 206 a - 206 d .
- the coaxial cable 204 is used to provide power and may also be used to provide one or more bi-directional communication channels to the GA-ISUs 206 a - 206 d.
- the GA-ISUs 206 a - 206 d are implemented to include one or more antenna elements integrated with one or more inertial sensing units. Exemplary GA-ISUs 206 are shown in FIG. 3A and FIG. 3B .
- GA-ISUs 206 according to embodiments of the present invention include inertial sensing units distributed to reside in close proximity to the phase center of the antenna unit. In embodiments in which a single antenna element is used, the phase center of the antenna unit is the phase center of the single element. In embodiments in which a plurality of elements are provided, the phase center is the geometric mean of the plurality of antenna elements.
- the inertial sensing reference is coincident with the GNSS sensing reference.
- various components of the GA-ISU 106 are mounted on a printed circuit board 310 to which are mounted an inertial sensing unit 308 , which may be implemented as a micro-electro-mechanical (MEMs) inertial sensing unit, and associated conditioning electronics.
- the inertial sensing unit 308 may be configured, for example, to provide three independent axes of acceleration and three independent axes of angular rate sensing.
- Suitable inertial sensing units are known and may include, for example, angular gyros, accelerometers. Additional sensors such as barometric, temperature or magnetic sensors may also be included.
- An antenna element 304 is provided in close proximity to the inertial sensing unit 308 .
- the phase center 306 of the antenna element 304 is substantially co-located with, or substantially adjacent, the inertial sensing unit 308 , for example, “substantially adjacent” means within no more than one-half wavelength of the highest frequency GNSS carrier frequency. In the case of GPS, the highest frequency is 1575.42 MHz, yielding a wavelength of 19.2 cm. More preferably, the inertial sensing unit 308 is no more than one-eighth to one-quarter wavelength from the phase center 306 and, in embodiments employing the GPS system, the phase center 306 and inertial sensing unit are most preferably within one centimeter of each other. In one embodiment, the inertial sensing unit 308 may be positioned on the opposite side of the printed circuit board 310 from the antenna element 304 .
- the inertial sensing components are distributed to the GNSS antenna units, where they directly measure the inertial environment of the antenna phase center.
- Low noise amplification (LNA) electronics may be provided in the housing 302 , typically coupled to the board 310 .
- Each GA-ISU 206 may further include a temperature sensor 312 to calibrate the repeatable thermal effects on the sensor package.
- an RF coaxial cable 204 for coupling the GA-ISU 206 to the GPS-IMU controller 202 ( FIG. 2A ).
- certain embodiments may also be provided with a separate communications channel 316 , for transmission of inertial sensor signals, as will be explained in greater detail below.
- the output of the GA-ISU 206 is communicated to the GPS-IMU processor 202 where it is combined with measurement data from the GNSS antenna(s).
- the RF coaxial cable 204 itself is used to transmit both the GNSS sensor data and the inertial sensor data.
- the inertial sensor signals may be modulated onto the cable using any of a variety of known modulation techniques, such as TDMA, CDMA, FDMA, etc. These signals are demodulated at the GPS-IMU 202 and processed with the GNSS phase data.
- FIG. 3B An alternate embodiment of a GA-ISU suitable for use in a positioning-navigation system according to embodiments of the present invention is shown in FIG. 3B .
- FIG. 3B illustrates GA-ISU 206 - 1 having multiple antenna elements 304 b - 1 , 304 b - 2 .
- an antenna inside of which is positioned a rigid printed circuit board 310 b , having inertial sensing unit 308 b .
- multiple antenna elements 304 b - 1 , 304 b - 2 are mounted on the printed circuit board 310 b .
- the antenna elements 304 b - 1 , 304 b - 2 are typically mounted in a symmetrical pattern such that the inertial sensor package 308 b may be mounted substantially at the geometric mean 350 of the phase centers 306 b - 1 , 306 b - 2 of the antenna elements 304 b - 1 , 304 b - 2 .
- Rigidity of the printed circuit board is important to maintain the relative positioning of the attached antenna elements.
- substantially at the geometric mean refers to placement within no more than one-half wavelength of the highest frequency GNSS carrier frequency..
- the inertial sensing unit 308 b is no more than one-eighth to one-quarter wavelength from the phase center 306 and, in embodiments employing the GPS system, the phase center 306 and inertial sensing unit are most preferably within one centimeter of the antenna unit phase center, i.e., the geometric mean 350 of the antenna element phase centers. It is noted, however, that other embodiments of the present invention may include multiple GNSS antenna elements associated in a same antenna unit with multiple, typically non-redundant, inertial sensor units 308 b .
- An inertial sensor unit is associated with an antenna unit if the relative motion of the inertial sensor unit and the antenna unit are substantially constrained by rigid body dynamics to within a fraction of a carrier cycle.
- GPS-IMU 202 receives GNSS signals and inertial measurement signals from the various GA-ISUs 206 a - 206 d , respectively.
- the GPS-IMU 202 is coupled to receive GNSS signals from the antenna 304 .
- the GPS-IMU 202 also receives the inertial measurement signals via a communications channel 407 .
- an RF coaxial cable may be used to provide the channel.
- the GNSS signals are provided to a preamplifier and downconverter 404 , which receives a clock signal from clock oscillator 412 .
- the signal is provided to a demodulator/phase extractor 406 , where the signal is mixed down to reference frequency with locally generated C/A or P code and demodulated. Also, the carrier phase may be extracted.
- the results are provided to a processing unit 415 .
- the processing unit 415 may be implemented as one or more suitably programmed processors or application specific integrated circuits (ASIC).
- the processing unit 415 includes a GNSS processing unit 414 and an IMU processing unit 416 .
- the GNSS processing unit 414 receives the GNSS message(s), code measurement(s) and time measurement(s) from the various input channels. From these, the GNSS signal processing unit 414 can perform data decoding, determination of satellite positions, pseudo-range calculations, and make determinations of receiver position, velocity and time.
- An exemplary system and method for handling GPS signals is described in copending, commonly-assigned U.S. patent application Ser. No. 10/408,496, titled “Satellite Navigation System Using Multiple Antennas,” which is hereby incorporated by reference in its entirety as if fully set forth herein.
- the GNSS signal processing unit 414 can also determine attitude and angular velocity.
- the GNSS signal processing unit 414 can also operate in conjunction with the IMU processing unit 416 to make inertial-based corrections of the GNSS position, etc., determinations.
- the IMU processing unit 416 receives, e.g., accelerometer and angular-rate sensor data from the GA-ISUs 106 .
- This data can be used to make independent measurements of changes in vehicle position, velocity, attitude and angular rate, or can be used in conjunction with the corresponding GNSS data to make an “enhanced” determination.
- the GNSS signal processing unit 414 can determine if the received GNSS data is valid; if the GNSS data from one or more of the GNSS channels is missing or corrupted (e.g., due to carrier phase cycle slip), then data from the inertial sensors can be used. Alternatively, the data from the inertial sensors can be used to calibrate the GNSS. Similarly, if the data from the inertial sensing units is determined to be invalid, then GNSS data can be used.
- FIG. 5 An alternate embodiment of a positioning-navigation system according to the present invention is shown in FIG. 5 .
- the GNSS receiver portion of the GPS-IMU 202 ( FIG. 4 ) is divided into a computational unit 503 and a plurality of signal tracking units 507 a - 507 d , distributed with the GA-ISUs 506 a - 506 d.
- the GPS-IMU 502 includes an IMU processor 505 similar to that of the GPS-IMU 102 of FIG. 4 , and a GNSS computational unit 503 .
- an external reference oscillator 512 may be provided that provides a common clock signal to the GA-ISUs 506 .
- the reference oscillator may be on-board the GPS-IMU 502 .
- FIG. 6 is a block diagram illustrating the GNSS receiver of the embodiment of FIG. 5 in greater detail. Shown are a plurality of signal tracking units 507 a - 507 d , which are distributed in the various GA-ISUs 506 a - 506 d ; for sake of simplicity, only signal tracking unit 507 a will be discussed.
- the signal tracking units 507 include preamplifier and downconverter 504 and demodulation/carrier phase extracting unit 506 .
- a common clock 512 is provided. As noted above, the common clock signal may be provided from a separate clock oscillator 512 , or from one on-board the GPS-IMU 502 .
- the resulting navigation message and code measurement signals are provided to the computational unit 503 on the GPS-IMU 402 .
- the computational unit 503 then processes the GNSS information along with the received IMU information to generate position, velocity, etc., information for use by the navigation system.
- GNSS signals are received at one or more antennas.
- Inertial measurements using inertial sensing units at the antennas, are obtained in step 703 .
- GNSS and inertial measurement units are used in a complementary filtering algorithm, typically based on statistical least squares estimation such as a linear or nonlinear Kalman filter, to update a state vector estimate.
- the state vector will vary with the application depending on the configuration and flexibility of the vehicle, the number of attached GA-IMU's and other details of the physical modeling.
- the filter implementation also provides the ability to compare inertially-derived and GNSS-derived measurements using statistical innovations as shown in step 706 .
- the statistical innovations are compared within predetermined or statistically defined limits, allowing the acceptance or rejection of the measurement set. If the values are outside the limit(s), then value(s) from the particular device may be determined to be invalid. For example, a value may be determined to be invalid if the particular device (i.e., antenna or inertial sensor) is not functioning or gives inconsistent measurements.
- the GNSS measurement may be determined to be invalid or a cycle slip may be repaired. Alternatively, the rejected device may then be isolated and its values not used for further determinations.
- Valid values are then used, in step 710 , to derive velocity, attitude, angular rate, etc.
- Additional embodiments of the invention may be employed to determine flexibility of the body to which the antenna units are attached. For example, flexure of an aircraft wing by sensing relative motion among a set of antenna attached units.
Abstract
Description
- 1. Field of the Invention
- The present invention is directed generally to navigation systems and, particularly, to an improved system and method for position, velocity, orientation or angular rate sensing.
- 2. Description of the Related Art
- Global navigation satellite system (GNSS) sensors are used in vehicles such as aircraft to determine vehicle position, velocity, orientation (attitude), and angular rate.
- Use of GNSS position sensors to determine vehicle position is well known. GNSS position sensors can also be used to determine a velocity estimate by solving a set of range-rate equations or by smoothing (curve fitting) a set of position measurements.
- A GNSS position sensor typically includes an antenna and an RF coaxial cable coupling the antenna to a GNSS signal processing unit. The GNSS antenna generally includes an antenna element and associated filtering and amplification electronics. Position is sensed at the phase center of the antenna (typically close to the geometric center of the antenna element). Typically, GNSS sensing provides bandwidth up to about 10 Hz.
- GNSS orientation (attitude) sensors are also known (Orientation of a rigid body in space is defined by three (3) independent parameters. While various specifications of these parameters are possible, heading, pitch and roll are commonly used.).
- An exemplary GNSS
attitude sensing system 100 is shown inFIG. 1A . Typically, a GNSS attitude sensing system includes a plurality of GNSS antennas 102 a-102 d coupled viacoaxial cables 104 a-104 d to aGNSS receiver unit 106. The relative positions of the antennas are used to derive a vehicle orientation. In addition, a GNSS attitude sensing system can generate angular rate measurements by solving a set of range-rate equations or by smoothing (curve-fitting) a set of attitude measurements. - Generally, the antennas are attached to the receiver via coaxial cables. To eliminate signal-to-noise (SNR) losses in the coaxial cable, low-noise amplification (LNA) and filtering electronics are placed in the GNSS antennas 102 a-102 d. The coaxial cable is used to transmit power from the receiver electronics to the antenna LNA electronics and to transmit the bandlimited GNSS signal to the
receiver unit 106, where further amplification, filtering and signal processing is performed. - Inertial sensors, such as accelerometers and angular-rate sensors, may be used either alone or in conjunction with GNSS sensors to determine changes in position, velocity, orientation, and angular-rate. Change in position, for example, can be determined by twice integrating a set of accelerometer measurements; change in velocity can be determined by once integrating a set of accelerometer measurements. Similarly, an angular-rate sensor can directly measure angular-rate. The change in orientation can then be derived from integrating the angular rate measurement.
- Inertial sensors are typically deployed in an inertial measurement unit (IMU) that houses, for example, an accelerometer, angular-rate, temperature and related sensors, as well as associated power supply, sampling filtering, and computational electronics. The IMU is typically located close to the center of gravity of the vehicle; the mounting orientation within the vehicle is an important installation constraint.
- Returning to
FIG. 1A , thesystem 100 includes an exemplary IMU 108 positioned generally at the vehicle's center of gravity and remote from the GNSSreceiver electronics 106. In the system shown, the measurements from both the GNSS sensors and the inertial sensors are available for processing. The GNSS measurements can be used to calibrate the inertial instruments over time by updating estimates of inertial sensor parameters at the relatively slow GNSS update rate. However, in situations where the tracking of the GNSS signals is compromised by low SNR, extreme antenna acceleration, destructive multipath or similar interference, latency between the IMU processor and the GNSS processor will generally preclude calibrating in the reverse direction. - Alternatively, the IMU may be combined with the GNSS receiver electronics in a single enclosure. Such a configuration is shown in
FIG. 1 B . As shown, aprocessing unit 105 includes bothGNSS receiver electronics 106 a and IMU 108 a. This topology offers several advantages over the topology ofFIG. 1A . These include elimination of the communications harness between the IMU and GNSS receiver unit; reduction in communication latency and complexity; and synchronous sampling of GNSS and inertial measurements, allowing: high bandwidth inertial measurement flow into the GNSS tracking channels; low bandwidth processed GNSS measurements update inertial measurement parameters; and high bandwidth GNSS phase data are available for update of inertial instrument parameters. - However, such a topology also suffers from disadvantages related to the fact that the point at which the inertial sensors reside is physically remote from the phase centers of the antennas. The GNSS
receiver 106 senses position (phase information) at the phase centers of its antennas. The IMU samples and integrates its internal sensors at the physical location of the IMU. Because the antennas and the IMU enclosure are physically separated, a projection algorithm must be applied before the measurements can be compared for purpose of complementary filtering. The projection algorithm requires the vectors between the IMU and the antenna phase centers be accurately known. This requirement can be problematic because it mandates an installation calibration procedure that may be complex; the vectors may change over time; and the vectors may change during operation, e.g., due to the structural flexibility of the vehicle, or elements of the vehicle. - These and other drawbacks in the prior art are overcome in large part by a system and method according to embodiments of the present invention.
- A global navigation satellite system (GNSS) receiver system according to an embodiment of the present invention includes a processing unit; and one or more antenna units for receiving GNSS signals, each of the antenna units having a phase center; one or more inertial sensor units each positioned substantially adjacent said phase centers; and at least one communication channel between each antenna unit and said processing unit. In certain embodiments, the processing unit uses data from the one or more antenna units and data from the one or more inertial sensor units to determine at least one of a position, velocity, orientation, or angular rate of the object. In certain embodiments of the invention, the antenna units include a single antenna element, the inertial sensing unit being positioned substantially adjacent the phase center of the antenna element. In other embodiments, the antenna units include a plurality of antenna elements mounted on a substantially rigid substrate, and the inertial sensing unit is positioned substantially adjacent an antenna unit phase center, defined as the geometric mean of the phase centers of the antenna elements. In this context, rigidity of the substrate denotes relative motion between the antenna elements of less than about one hundredth of the highest frequency GNSS carrier wavelength during operation.
- An antenna unit for use in a global navigation satellite system (GNSS) receiver according to an embodiment of the present invention includes an integrated inertial sensor unit positioned substantially adjacent a phase center of the antenna unit. In certain embodiments, the phase center of the antenna unit is the phase center of a single antenna element. In other embodiments, the phase center of the antenna unit is the geometric mean (equally weighted centroid) of the phase centers of a plurality of antenna elements.
- The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference symbols in different drawings indicates similar or identical items.
-
FIG. 1A andFIG. 1B illustrate GNSS and IMU systems according to the prior art; -
FIG. 2A is a block diagram of an exemplary positioning and navigation system in accordance with an embodiment of the invention. -
FIG. 2B illustrates an exemplary object employing a positioning-navigation system according to an embodiment of the present invention. -
FIG. 3A andFIG. 3B illustrate exemplary antenna units according to embodiments of the present invention. -
FIG. 4 is a block diagram of an exemplary processing unit according to an embodiment of the present invention. -
FIG. 5 is a block diagram of an exemplary positioning and navigation system in accordance with an embodiment of the invention. -
FIG. 6 is a block diagram of an exemplary processing unit according to an embodiment of the present invention. -
FIG. 7 is a diagram illustrating operation of an embodiment of the present invention. - Turning now to
FIG. 2A a block diagram of an exemplary navigation-positioning system 201 in accordance with an embodiment of the present invention is shown. In particular, the system provides global navigation satellite system (GNSS) based and inertial-based determination of vehicle position, velocity, orientation (attitude) and angular rate. Inertial sensors are co-located with GNSS antennas to more accurately derive the desired position and rate information. - It is noted that, while in exemplary embodiments, the GNSS receives positioning signals from the Global Positioning System (GPS), the system may be used with other radio based positioning or navigation systems, such as the GLONASS system, Galileo, or other systems such as pseudolites, low earth orbiting satellites (LEO), geosynchronous satellites, etc.
- In the embodiment illustrated, the navigation-
positioning system 201 includes a plurality of antenna units embodied as integrated GNSS Antenna-Inertial Sensing Units (GA-ISU) 206 a-206 d. As will be explained in greater detail below, each GA-ISU 206 a-206 d combines one or more GNSS antenna elements for receiving GNSS signals with an inertial sensor for determining, e.g., vehicle position, velocity, orientation (attitude) and angular rate. It is noted that while four such units are shown, in practice the number may vary. Each GA-ISU 206 a-206 d is coupled to a GNSS Receiver-Inertial Measurement Unit (GPS-IMU)processor 202. The GPS-IMU processor 202 receives the GNSS antenna sensor data and the inertial sensor data, which it processes to derive the desired position, velocity, orientation and angular rate information. The resulting information is then provided to anavigation processor 203 for use, for example, in steering control or course guidance. -
FIG. 2B illustrates typical vehicle topology for a positioning-navigation system according to embodiments of the present invention. In particular, shown is avehicle 200, which may be embodied as an aircraft, although the invention is not so limited and may be implemented in any body whose position, orientation, flexibility, etc., is desired to be determined. For example, the body may be a substantially rigid body; a substantially flexible body; or a plurality of substantially rigid, connected, bodies articulated (or independently moving) with respect to one another. A body may be considered to be flexible if due to structural flexibility, relative motion between affixed antennas of greater than one tenth of the highest frequency GNSS carrier wavelength is possible during operation. Otherwise, a body may be considered to be rigid. - The GPS-
IMU 202 and GA-ISU's 206 a-206 d are fixed in suitable locations of thevehicle 200. Typically, the GPS-IMU 202 is coupled via one or morecoaxial cables 204 to the GA-ISUs 206 a-206 d. Thecoaxial cable 204 is used to provide power and may also be used to provide one or more bi-directional communication channels to the GA-ISUs 206 a-206 d. - As noted above, the GA-ISUs 206 a-206 d are implemented to include one or more antenna elements integrated with one or more inertial sensing units. Exemplary GA-ISUs 206 are shown in
FIG. 3A andFIG. 3B . In particular, GA-ISUs 206 according to embodiments of the present invention include inertial sensing units distributed to reside in close proximity to the phase center of the antenna unit. In embodiments in which a single antenna element is used, the phase center of the antenna unit is the phase center of the single element. In embodiments in which a plurality of elements are provided, the phase center is the geometric mean of the plurality of antenna elements. Thus, the inertial sensing reference is coincident with the GNSS sensing reference. - In the embodiment of the present invention shown in
FIG. 3A , various components of the GA-ISU 106 are mounted on a printedcircuit board 310 to which are mounted aninertial sensing unit 308, which may be implemented as a micro-electro-mechanical (MEMs) inertial sensing unit, and associated conditioning electronics. Theinertial sensing unit 308 may be configured, for example, to provide three independent axes of acceleration and three independent axes of angular rate sensing. Suitable inertial sensing units are known and may include, for example, angular gyros, accelerometers. Additional sensors such as barometric, temperature or magnetic sensors may also be included. - An
antenna element 304 is provided in close proximity to theinertial sensing unit 308. In particular, thephase center 306 of theantenna element 304 is substantially co-located with, or substantially adjacent, theinertial sensing unit 308, for example, “substantially adjacent” means within no more than one-half wavelength of the highest frequency GNSS carrier frequency. In the case of GPS, the highest frequency is 1575.42 MHz, yielding a wavelength of 19.2 cm. More preferably, theinertial sensing unit 308 is no more than one-eighth to one-quarter wavelength from thephase center 306 and, in embodiments employing the GPS system, thephase center 306 and inertial sensing unit are most preferably within one centimeter of each other. In one embodiment, theinertial sensing unit 308 may be positioned on the opposite side of the printedcircuit board 310 from theantenna element 304. - Thus, the inertial sensing components are distributed to the GNSS antenna units, where they directly measure the inertial environment of the antenna phase center. Low noise amplification (LNA) electronics (not shown) may be provided in the
housing 302, typically coupled to theboard 310. Each GA-ISU 206 may further include atemperature sensor 312 to calibrate the repeatable thermal effects on the sensor package. - Also shown is an RF
coaxial cable 204 for coupling the GA-ISU 206 to the GPS-IMU controller 202 (FIG. 2A ). In addition, certain embodiments may also be provided with aseparate communications channel 316, for transmission of inertial sensor signals, as will be explained in greater detail below. - The output of the GA-ISU 206 is communicated to the GPS-
IMU processor 202 where it is combined with measurement data from the GNSS antenna(s). In one embodiment, the RFcoaxial cable 204 itself is used to transmit both the GNSS sensor data and the inertial sensor data. For example, the inertial sensor signals may be modulated onto the cable using any of a variety of known modulation techniques, such as TDMA, CDMA, FDMA, etc. These signals are demodulated at the GPS-IMU 202 and processed with the GNSS phase data. - An alternate embodiment of a GA-ISU suitable for use in a positioning-navigation system according to embodiments of the present invention is shown in
FIG. 3B .FIG. 3B illustrates GA-ISU 206-1 havingmultiple antenna elements 304 b-1, 304 b-2. In particular, shown is an antenna inside of which is positioned a rigid printed circuit board 310 b, having inertial sensing unit 308 b. In addition, as noted above,multiple antenna elements 304 b-1, 304 b-2 are mounted on the printed circuit board 310 b. Theantenna elements 304 b-1, 304 b-2 are typically mounted in a symmetrical pattern such that the inertial sensor package 308 b may be mounted substantially at thegeometric mean 350 of the phase centers 306 b-1, 306 b-2 of theantenna elements 304 b-1, 304 b-2. Rigidity of the printed circuit board is important to maintain the relative positioning of the attached antenna elements. Again, “substantially at” the geometric mean refers to placement within no more than one-half wavelength of the highest frequency GNSS carrier frequency.. More preferably, the inertial sensing unit 308 b is no more than one-eighth to one-quarter wavelength from thephase center 306 and, in embodiments employing the GPS system, thephase center 306 and inertial sensing unit are most preferably within one centimeter of the antenna unit phase center, i.e., thegeometric mean 350 of the antenna element phase centers. It is noted, however, that other embodiments of the present invention may include multiple GNSS antenna elements associated in a same antenna unit with multiple, typically non-redundant, inertial sensor units 308 b. An inertial sensor unit is associated with an antenna unit if the relative motion of the inertial sensor unit and the antenna unit are substantially constrained by rigid body dynamics to within a fraction of a carrier cycle. - An exemplary GNSS Receiver-Inertial Measurement Unit (GPS-IMU)
controller 202 according to an embodiment of the invention is shown inFIG. 4 . In the embodiment illustrated, GPS-IMU 202 receives GNSS signals and inertial measurement signals from the various GA-ISUs 206 a-206 d, respectively. For sake of simplicity, only oneinput channel 409 is shown inFIG. 4 , it being understood that inputs from the remaining GA-ISU units are handled similarly (These are represented inFIG. 4 by reference numeral 403). Thus, the GPS-IMU 202 is coupled to receive GNSS signals from theantenna 304. The GPS-IMU 202 also receives the inertial measurement signals via acommunications channel 407. As noted above, an RF coaxial cable may be used to provide the channel. - The GNSS signals are provided to a preamplifier and
downconverter 404, which receives a clock signal fromclock oscillator 412. The signal is provided to a demodulator/phase extractor 406, where the signal is mixed down to reference frequency with locally generated C/A or P code and demodulated. Also, the carrier phase may be extracted. The results are provided to aprocessing unit 415. Theprocessing unit 415 may be implemented as one or more suitably programmed processors or application specific integrated circuits (ASIC). - In the embodiment illustrated, the
processing unit 415 includes aGNSS processing unit 414 and anIMU processing unit 416. TheGNSS processing unit 414 receives the GNSS message(s), code measurement(s) and time measurement(s) from the various input channels. From these, the GNSSsignal processing unit 414 can perform data decoding, determination of satellite positions, pseudo-range calculations, and make determinations of receiver position, velocity and time. An exemplary system and method for handling GPS signals is described in copending, commonly-assigned U.S. patent application Ser. No. 10/408,496, titled “Satellite Navigation System Using Multiple Antennas,” which is hereby incorporated by reference in its entirety as if fully set forth herein. - From the data from the multiple sensors, the GNSS
signal processing unit 414 can also determine attitude and angular velocity. The GNSSsignal processing unit 414 can also operate in conjunction with theIMU processing unit 416 to make inertial-based corrections of the GNSS position, etc., determinations. - Thus, as noted above, the
IMU processing unit 416 receives, e.g., accelerometer and angular-rate sensor data from the GA-ISUs 106. This data can be used to make independent measurements of changes in vehicle position, velocity, attitude and angular rate, or can be used in conjunction with the corresponding GNSS data to make an “enhanced” determination. - For example, the GNSS
signal processing unit 414 can determine if the received GNSS data is valid; if the GNSS data from one or more of the GNSS channels is missing or corrupted (e.g., due to carrier phase cycle slip), then data from the inertial sensors can be used. Alternatively, the data from the inertial sensors can be used to calibrate the GNSS. Similarly, if the data from the inertial sensing units is determined to be invalid, then GNSS data can be used. - An alternate embodiment of a positioning-navigation system according to the present invention is shown in
FIG. 5 . In particular, in thesystem 500 ofFIG. 5 , the GNSS receiver portion of the GPS-IMU 202 (FIG. 4 ) is divided into acomputational unit 503 and a plurality of signal tracking units 507 a-507 d, distributed with the GA-ISUs 506 a-506 d. - In this embodiment, the GPS-
IMU 502 includes anIMU processor 505 similar to that of the GPS-IMU 102 ofFIG. 4 , and a GNSScomputational unit 503. In addition, anexternal reference oscillator 512 may be provided that provides a common clock signal to the GA-ISUs 506. In other embodiments, the reference oscillator may be on-board the GPS-IMU 502. -
FIG. 6 is a block diagram illustrating the GNSS receiver of the embodiment ofFIG. 5 in greater detail. Shown are a plurality of signal tracking units 507 a-507 d, which are distributed in the various GA-ISUs 506 a-506 d; for sake of simplicity, only signal trackingunit 507 a will be discussed. The signal tracking units 507 include preamplifier anddownconverter 504 and demodulation/carrierphase extracting unit 506. In addition, acommon clock 512 is provided. As noted above, the common clock signal may be provided from aseparate clock oscillator 512, or from one on-board the GPS-IMU 502. - The resulting navigation message and code measurement signals are provided to the
computational unit 503 on the GPS-IMU 402. Thecomputational unit 503 then processes the GNSS information along with the received IMU information to generate position, velocity, etc., information for use by the navigation system. - Turning now to
FIG. 7 , a diagram illustrating operation of an embodiment of the present invention is shown. Initially, at astep 702, GNSS signals are received at one or more antennas. Inertial measurements, using inertial sensing units at the antennas, are obtained instep 703. In astep 704, GNSS and inertial measurement units are used in a complementary filtering algorithm, typically based on statistical least squares estimation such as a linear or nonlinear Kalman filter, to update a state vector estimate. The state vector will vary with the application depending on the configuration and flexibility of the vehicle, the number of attached GA-IMU's and other details of the physical modeling. The filter implementation also provides the ability to compare inertially-derived and GNSS-derived measurements using statistical innovations as shown instep 706. In astep 708, the statistical innovations are compared within predetermined or statistically defined limits, allowing the acceptance or rejection of the measurement set. If the values are outside the limit(s), then value(s) from the particular device may be determined to be invalid. For example, a value may be determined to be invalid if the particular device (i.e., antenna or inertial sensor) is not functioning or gives inconsistent measurements. In other cases, for example, in the event a carrier phase cycle slip is detected, the GNSS measurement may be determined to be invalid or a cycle slip may be repaired. Alternatively, the rejected device may then be isolated and its values not used for further determinations. Valid values are then used, instep 710, to derive velocity, attitude, angular rate, etc. Additional embodiments of the invention may be employed to determine flexibility of the body to which the antenna units are attached. For example, flexure of an aircraft wing by sensing relative motion among a set of antenna attached units. - The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The drawings and description were chosen in order to explain the principles of the invention and its practical application. The drawings are not necessarily to scale and illustrate the device in schematic block format. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents
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